A drain valve of this type has been known from, e.g., CH-A-386,351. The housing, called a bell there, is inserted into an opening in the bottom of the flush tank and is firmly screwed together with same. The float, which is rigidly connected to the overflow pipe, is accommodated in a circular cylindrical space, which is open at the bottom, under a partition or cover wall of the bell. The overflow pipe forms a closing member with a valve disk arranged at the lower end. To trigger a flushing, the closing body or the overflow pipe with the float fastened thereto is raised until the float comes into contact with the underside of the partition. The valve disk of the closing member is thus lifted off from the valve seat and the valve is thus opened. Flushing water present in the flushing tank flows through lateral windows in the bell into, e.g., a connected toilet bowl. The drain valve closes as soon as air enters into the area under the partition through a gap between the overflow pipe and the bell. The mounting of this drain valve is not quite simple, because the valve tube with the valve disk must be inserted into the bell from below. This is difficult especially when means for controlling the rate of lowering are to be mounted in the drain valve.